1. Field of the Invention
This invention relates to a novel polymeric thin film, a process for producing said thin film by utilization of plasma polymerization and products containing said thin film.
2. Description of the Prior Art
There have been known processes for preparing a polymeric thin film by providing a saturated or unsaturated hydrocarbon gas for plasma polymerization [H. Yasuda, Contemporary Topics in Polymer Science, Vol. 3, 103 (1979), Ed. by M. Shen, Plenum Publishing Corp.]. The polymeric thin films obtained by these processes, which are higher in crosslinking density and also greater in mechanical strength than the conventional plastics, promise to be utilizable for surface protective films or surface hardening films on various kinds of articles. However, in some uses, these thin films are insufficient in hardness and toughness and therefore cannot be utilized as protective films, etc. For example, it has been attempted to form a plasma polymerized film as the protective film on a magnetic recording medium such as video tape comprising a magnetic metal thin film provided on a non-magnetic support. Such a magnetic recording medium is subjected to frictional contact with a magnetic head, a guide pole, etc. during running and therefore the protective film therefor is required to have a high scratch resistance so that scratches will not be readily formed. Also, particularly in the case of a video tape, mechanical impact is applied repeatedly to the magnetic recording medium during reproduction of still images, so that it is required to have high durability. If the durability is low, the still life time of the magnetic recording medium, which is the time after initiation of reproduction of still image until the whole image disappears due to noise, will become short. However, where the polymeric thin film as mentioned above is formed on a recording medium, the resultant recording medium is low in scratch resistance and short in still life because said polymeric thin film is low in both hardness and toughness.
Most of the plasma polymerized films from hydrocarbons have densities of 1 to 1.5 gm/cm.sup.3, but it is known in the art that, depending on the reaction conditions, plasma polymerization of methane can give a thin film of fairly high hardness [A. Doi et al, Proc. Int'l Ion Engineering Congress--ISIAT '83 & IPAT '83, KYOTO (1983), page 1137]. We experimentally confirmed that the plasma polymerized film of methane may have a density higher than 1.5 gm/cm.sup.3. However, because of low toughness, the thin film thus prepared is brittle and liable to crack and is therefore unsuitable as a protective film for a product such as a magnetic recording medium which is subjected to harsh conditions. Further, in this case, there is also the drawback that the deposition rate of the thin film is low so that a long time is required for production.
Japanese Laid-open Patent Publication No. 180503/1983 discloses a process for preparing a thin film by carrying out plasma polymerization by use of at least one mixed monomer gas selected from (A)+(B), (A)+(C), (A)+(B)+(C) and (B)+(C), constituted by selection from halogenated hydrocarbon gas (A), hydrocarbon gas (B), hydrogen gas (C) and halogen gas (D), wherein the electron temperature at the site where plasma polymerization is carried out is controlled to 30,000.degree.K. to 90,000.degree.K. and the atomic ratio of halogen atoms to hydrogen atoms in the mixed monomer gas is controlled within a range of from 1:2 to 5:1. Although the thin film obtained according to this process has high hardness, it too is low in toughness and is therefore unsuitable as a protective film for a magnetic recording medium for the same reason as mentioned above regarding the plasma polymerized film obtained from methane.